The use of a hand operated pointing device for use with a computer and its display has become almost universal. One form of the various types of pointing devices is the conventional (mechanical) mouse, used in conjunction with a cooperating mouse pad. Mechanical mice typically include a rubber-surfaced steel ball that rolls over the mouse pad as the mouse is moved. Interior to the mouse are rollers, or wheels, that contact the ball at its equator and convert its rotation into electrical signals representing orthogonal components of mouse motion. These electrical signals are coupled to a computer, where software responds to the signals to change by a ΔX and a ΔY the displayed position of a pointer (cursor) in accordance with movement of the mouse.
In addition to mechanical types of pointing devices, such as a conventional mechanical mouse, optical pointing devices have also been developed. In one form of an optical pointing device, rather than using a moving mechanical element like a ball, relative movement between an imaging surface, such as a finger or a desktop, and photo detectors within the optical pointing device, is optically sensed and converted into movement information. Battery operated optical mice are currently available based on Agilent's ADNS-2020 and ADNS-2030 optical image sensors. Other optical mice are available based on Agilent's ADNS-2001 and ADNS-2051, as well as other optical image sensors.
Limiting the power consumed by optical pointing devices is important for portable electronic devices, such as portable computers, cellular telephones, personal digital assistants (PDA's), digital cameras, portable game devices, pagers, portable music players (e.g., MP3 players), and other similar devices that might incorporate an optical pointing device. Limiting power consumption is also important for wireless optical pointing devices, such as wireless optical mice.
One major source of power drain in optical pointing devices is the light source typically used in these devices. For an optical mouse, the light source, such as a light emitting diode (LED), illuminates the surface under the mouse. While the mouse is moved, the LED is typically turned on at a constant frequency based on the frame rate of the optical motion sensor. Several techniques have been developed to reduce the power drain caused by the light source. For example, some optical motion sensors for optical pointing devices include a low-power or “sleep” mode that is automatically entered if no motion is detected for a period of time. In low power mode, power savings is achieved by turning off the light source of the optical pointing device, or turning the light on less frequently than in full power mode. In low power mode, the sensor typically does not measure how much motion occurs, but rather determines whether any motion has occurred. When motion is detected, the sensor reverts back to a full power mode with a higher frame rate appropriate for tracking motion.
In the low power mode in some optical motion sensors, images are captured, but at a significantly reduced rate compared to the rate at which images are captured in the full power mode. Some optical motion sensors provide 1500 or more “frame periods” per second. An image may or may not be captured during a frame period. For example, in full power mode, an image may be captured during each frame period, resulting in 1500 images per second. In low power mode, an image may only be captured every 10 or 12 frame periods, resulting in 125–150 images per second, or an image may only be captured every 750 to 1500 frame periods, resulting in one to two images per second.
In the full power mode of some optical motion sensors, the light source remains on for all frame periods, and is not turned off during a frame period or between frame periods. In the low power mode of some optical motion sensors, the light source is turned on only during frame periods when images are captured. Optical motion sensors have also been developed that adjust the pulse width of the light source drive signal to only turn the light source on during the integration phase of a frame period to save additional power.
In the full power mode of some optical mice, 1500 or more images are captured per second, which allows the mice to be able to track high velocity movements (e.g., 0.5 meters per second). The frame rate (e g., 1500 frames per second) in the full power mode of these mice is constant, regardless of the speed that the mouse is moved. However, the majority of mouse movements are not at high velocities, and slower movements can be accurately tracked at a slower frame rate. Excess power is consumed by tracking these slower movements at a high frame rate.
In the low power mode of some optical mice, even though a lesser number of images are typically captured per second than in a full power mode, the optical motion sensors in these devices still typically perform essentially the same analog-to-digital conversion and digital signal processing functions on the full image frames as those performed during full power mode. A relatively significant amount of power is consumed by digitizing and processing these full frames to determine whether any motion has occurred.